Read-write magnetic transducer having a composite structure comprising a stack of thin films

ABSTRACT

Each pole-piece of a read-write magnetic transducer having a geometrical airgap defined by a pair of pole-pieces and wherein passes a part of a read-write flat winging comprises a first layer made of magnetically soft material adjacent to the airgap and a second layer made of magnetically hard material with an intervening layer of a structure ensuring de-coupling of the magnetizations of the said soft and hard magnetic material layers.

United States Patent [191 Lazzari [451 Feb. 18, 1975 1 1 READ-WRITE MAGNETIC TRANSDUCER HAVING A COMPOSITE STRUCTURE COMPRISING A STACK OF THIN FILMS {75] Inventor: Jean-Pierre Lazzari, Villiers Saint Frederic, France [73] Assignee: Compagnie Internationale Pour LInformatique, Louveciennes, France 22 Filed: on. 25, 1973 211 App]. No.: 409,613

[30] Foreign Application Priority Data Nov. 7, 1972 France 72.39317 [52] US. Cl 360/126, 360/122, 360/125 [51] Int. Cl. ..G1lb 5/12, G1 1b 5/22 [58] Field of Search 179/1002 C; 340/1741 F; 346/74 MC; 29/603; 360/122, 123, 125, 126

[56] References Cited UNITED STATES PATENTS 3,344,237 9/1967 Gregg 179/1002 C 3,549,825 12/1970 Trimble 179/1002 C 3,564,558 2/1971 Tolman et a1. 179/1002 C 3,611,417 10/1971 Sauter et a1 179/1002 C 3,639,699 2/1970 Tiemann 179/1002 C 3,700,827 10/1972 Nagao 179/1002 C 3,723,665 3/1973 Lazzari et al. 179/1002 C Primary Examiner-Alfred H. Eddleman Attorney, Agent, or Firm-Kemon, Palmer & Estabrook [57] ABSTRACT Each pole-piece of a read-write magnetic transducer having a geometrical airgap defined by a pair of polepieces and wherein passes a part of a read-write flat winging comprises a first layer made of magnetically soft material adjacent tothe airgap and a second layer made of magnetically hard materialawith an intervening layer of a structure ensuring de-coupling of the magnetizations of the said soft and hard magnetic material layers.

8 Claims, 5 Drawing Figures PAIEMIEU FEB I 81975 FlG.2

FIG .1

1 READ-WRITE MAGNETIC TRANSDUCER HAVING A COMPOSITE STRUCTURE COMPRISING A STACK OF THIN FILMS BRIEF SUMMARY OF THE INVENTION The present invention concerns improvements in or relating to the read-write magnetic transducers made in layers wherein a flat coil winding is partially inserted between two magnetic flat pole-pieces defining a geometrical airgap therebetween. The magnetic circuit of the pole-pieces carries the field which, for a writing operation, is generated by the coil winding and during a read-out operation, said circuit carries the field generated from the magnetization variations of a record carrier.

In read-out operation, the efficiency of the transducer may be defined by the ratio of the magnetic flux crossed by the coil winding to the magnetic flux available at the airgap level: the reluctance of the magnetic material ought to be in such a condition as low as possible from the winding to the airgap and, for a fair definition of the read-out signal, the length of the airgap ought to be the minimum one.

In write-in operation, on the other hand, the efficiency of the transducer is related to the ratio of the writing field to the writing electrical current and of the gradient of the magnetic field at the airgap level: the magnetic material ought to have a substantially zero remanent flux density and, on the other hand, a maximal value saturation flux density, together with a permeability value well above 50.

These two sets of physical conditions have small compatibility, not to say they are contradictory. They led the manufacturers to separately design read-only magnetic transducers and write-only magnetic transducers.

In view of tentatively obviating such a contradiction, it has been proposed, for instance in U.S. Pat. No. 3,639,699 issued Feb. 1, 1972 in the name of 1.1. TIE- MAN, to provide each leg or pole-piece of the magnetic circuit comprised by two layers which are directly contacting one another and which are of magnetic materials of different properties: the magnetic material of the layer closest to the airgap has a substantially higher permeability and lower saturation flux density than the other layer. This design however suffers a major drawback. As the magnetic layers are directly contacting, the magnetization of the inner layer is blocked by the magnetization of the outer layer so that, in actual practice, the higher permeability quality of the inner layer is entirely lost.

It is the object of the invention to provide a magnetic read-write transducer structure which enables in a very simple way the efficiency and actuality of this two-layer pole-piece suggestion.

According to a feature of the invention, this aim is reached by providing between a magnetically soft inner layer and a magnetically hard outer layer of a polepice, an intervening layer of such a structure that it provides tic-coupling of the magnztizations of the said two layers.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in full detail with reference to the accompanying drawings wherein:

FIG. 1 shows an exploded view of a magnetic transducer structure according to the invention and from LII which may be plainly derived any modification within the field and scope of the appended claims,

FIG. 2 shows a top view of the structure,

FIGS. 3 and 4 show cross-section views along section lines a) and b) of FIG. 2, and,

FIG. 5 shows a partial cross-section view of enlarged scale, of an illustrative embodiment of one of the pole piece ofa transducer made in accordance to the invention.

In the drawings, the relative dimensionings of the layers, and mainly the thicknesses of said layers are not maintained for the sake of clarity. For the sake of clarity, too, the actual deformations of the layers are not indicated in the exploded view of FIG. 1, Such deformations normally occur during deposition in vacuo of the successive layers of the structure, according to a well known technique.

DETAILED DESCRIPTION Referring to FIG. 1, a layer 2 of a magnetic material of high saturation induction and high saturation flux density and of a substantially zero remanent flux density, and of a permeability much higher than 50, is coated over a substrate 1. The material of this layer may be cobalt or an iron-cobalt alloy or, preferably, pure iron which has a saturation flux density higher than that of cobalt or of the iron-cobalt alloys. Iron can be evaporated at temperatures compatible with these of the general process of evaporation of the layers of the structure. The remanent induction of iron is quite small and practically negligible with respect of that of the hard material which will be later used in the structure. The saturation fiux density of evaporated iron substantially equals 20,000 gauss whereas the corresponding parametric value is about 18,000 for cobalt. The permeability of iron, when evaporated in a thin layer or film at a substrate temperature of about 200 to 300 C. with a thickness of about twenty microns for instance, is from to 300. The saturation field of the layer, which can be considered as corresponding to the anisotropy field of a uniaxial layer for the purpose of the invention, may reach and even outpass 25 Oersteds for identical geometrical dimensions.

Such a layer 2 is coated with a layer 3 the structure of which will be hereinbelow defined. Over said layer 3 is coated a layer 4 of a magnetic material having, with respect to the material of the layer 2, a lower saturation flux density, an appreciably higher permeability and a value of anisotropy field appreciably lower than the saturation field value of the layer 2. Illustratively, the material of the layer 4 is an iron-nickel alloy having a saturation field density of the order of 10,000 gauss, a permeability value from 5000 to 1000 and an anisotropy field value of about 7 Oersteds, when evaporated to a substantially identical thickness as the layer 2.

A flat coil winding 5 is formed over the above stack of layers which, together, constitute one of the polepieces of the magnetic circuit of the transducer. As presently wellknown, such a flat coil winding is made of a flat multi-turn helix of relatively insulated conductors. The winding is applied by its front branch over the stack of layers 2 to 4 and directly applies on the substrate l by its lateral and rear branches of legs. Once formed, it is completely coated with an insulating film 9, FIG. 4 except its front face which is level to the airgap. In said front face at least one insulated conductor of the winding is bare. The airgap faces of the layers 2 to 4 and of the winding 5 are level with an edge of the substrate 1. The length L3 of said edge equels the length Ll of the layers 2 and 3 plus twice the width A of the winding legs. The length L2 of the layer 4 is preferably made lower than L1.

The thickness of the winding at its airgap face defines a geometrical airgap for the transducer. The stack 2-3-4 constitutes one of the pole-pieces of the magnetic circuit of the transducer and the second polepiece is formed over the winding and the stack in symmetrical relation to the first, i.e., a layer 6 of same material and geometry as the layer 4, a layer 7 of same structure and geometry as the layer 3 and the layer 8 of same material and geometry as layer 2. Illustratively though not imperatively, small blocks such as shown at 11 in FIG. 4, may be formed in insulating non-magnetic material of same thickness as the layer 6 on either side of said layer over the front branch of the winding 5. The composite layer 6-ll then provides a flat surface for deposition of the layers 7 and 8 at this location of the structure. When such blocks 11 are omitted, the layers 7 and 8 will reach the lateral edges of the layer 6 up to the level of the top surface of the winding airgap leg.

The structure of intervening layers 3 and 7 is such that they create a magnetostatic decoupling between the layers 2 and 4, respectively between the layers 6 and 8, so that the magnetizations of the layers 4 and 6 cannot be disturbed by the magnetization of the layers 2 and 8. Layers 2 and 8 are made of magnetically hard material as apparent from the above whereas layers 4 and 6 are made of magnetically soft material and, for an efficient operation of the head comprised of said transducer structure, it is imperative that the magnetizations of the soft layers cannot be influenced by the magnetizations of the hard layers so that the soft layers may preserve their easy magnetization axis parallel to the breadth of the airgap.

The decoupling layers 3 and 7 may each consists of a nonmagnetic dielectric material such for instance as Si with a thickness ensuring the required de-coupling of the above defined magnztizations. As a modification, each of the said layers 3 and 7 may consists of a composite structure comprised of alternate thin layers or films of soft and hard materials. The individual thickness of such thin films is much lower than the thickness of the layers between which they intervene, i.e., the layers 2 and 4 and the layers 6 and 8. In such a decoupling composite layer, the thin films of magnetically hard material duly block the magnetizations of the thin films of magnetically soft material but the structure does not present any leakage magnetic field nor any demagnetizing fields capable of influencing the layers between which it is inserted, consequently ensuring the required decoupling between said layers.

The second structure for the layer 3 is shown in FIG. wherein the thin layers 14, assumed of magnetically soft material, alternate with the layers 15, assumed of magnetically hard material.

In FIG. 5 is further shown a modification of the structures of the layers 2 and 4. Instead of forming such layers as monolithic layers, layer 2 is made ofa plurality of superposed relatively insulated thin films 12 and layer 14 is similarly made of a plurality of superposed relatively insulated thin films 13. The material of the thin films 12 is a magnetically soft one. The material of the thin films 13 is a magnetically hard one. The relative insulation of the thin films is obtained by deposition of very thin insulating films between the thin magnetic films. The purpose of such an arrangement, which may, if desired, be provided only for the magnetically 5 soft material layers, is to enable an optimalization of the dimensioning of the transducer, mainly of the overall thickness of said transducer, as it eliminates the effect of demagnetizing fields which, in monolithic layers" will have their paths imperatively closing through IO the hard material layers in which they would sink through the air from the soft material layers which generate such demagnetizing fields. Such demagnetizing fields would create extraneous couplings of the magnetizations of the soft and hard material layers, consel5 quently enhancing the possibilities of blocking the magnetization of the inner layers of the pole-pieces unless exageration of the thickness of the decoupling layers in said pole-pieces.

With a structure of transducer such as above desccibed and shown, the layers 4 and 6 will have negligible action during a write-in operation whereas the action of the layers 2 and 8 will be efficient to define a width (e FIG. 4) of the magnetic airgap. The recording fields will duly penetrate within the recording magnetic layer moving along the direction of the arrow S at close proximity of the airgap face of the transducer. On the other hand, the action of the layers 2 and 8 will be nil during a read-out operation from such a moving record and the layers 4 and 6 will define the width of the read airgap (e,) which is substantially equal to the width of the geometrical airgap e.

The lateral lengths of the soft and hard material layers have been shown different as it is thought preferable to read on a reduced length of the magnetic record with respect to the length over which the record has been written. However, such a dimensioning is not deemed imperative per se so that lengthes L2 and L1 may be made equal if desired.

It may conclusively be stated that the magnetic transducer structures according to the invention have their read and write functions duly separated as the magnetically soft material layers are relatively tightly coupled and do not have any substantial leakage field therefrom and that, further, said soft material layers are decoupled from any magnetostatic damageable effect from the hard material layers and are consequently protected against the leakage fields from the said hard material layers.

What is claimed is:

1. A read/write transducer comprising: a pair of magnetic pole-pieces and read-write conductor means inductively coupled to said pole-pieces and passing therebetween; each pole-piece including the superposition of:

a first magnetic layer member close to said conductor means; a magnetostatically decoupling layer means, over said first magnetic layer member; and a second magnetic layer member of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anistropy field than said first magnetic layer member. 2. A read/write transducer according to claim 1, wherein said magnetostatically decoupling layer means is comprised of a non-magnetic dielectric material.

3. A read/write transducer according to claim 1, wherein said magnetostatically decoupling layer means comprises a stack of thin films of alternating magnetic materials one of which is of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anisotropy than the other one.

4. A read/write transducer according to claim 1, wherein said first magnetic layer member consists of a stack of relatively insulated thin magnetic films.

5. A read/write transducer according to claim 4, wherein said second magnetic layer member also consists of a stack of relatively insulated thin magnetic films.

6. A read/write transducer according to claim 1, wherein the material of said first magnetic layer member is an iron-nickel alloy and the material of said second magnetic layer member is selected from the group consisting of iron, cobalt and the iron-cobalt alloys.

7. In a read/write magnetic transducer comprising a pair of magnetic pole-pieces and read-write conductor means inductively coupled to the pole-pieces and passing therebetween, a composite pole-piece structure comprising a stack of magnetic material thin films wherein a first plurality of said films close to the said conductor means are relatively insulated and of a first magnetic material, a second plurality of said films over the first plurality are directly contacting each other and alternate first and second magnetic materials, and a third plurality of said films are relatively insulated and of the said second magnetic material over the second plurality of thin films, and wherein the said second magnetic material is of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anisotropy field than the first magnetic material.

8. In a read/write magnetic transducer comprising a pair of magnetic pole-pieces and read-write conductor means inductively coupled to the pole-pieces and passing therebetween, a composite pole-piece structure comprising a first plurality of relatively insulated thin film of a first magnetic material close to said read-write conductor means, a nonmagnetic dielectric material layer over first plurality of thin films, and a second plurality of relatively insulated thin films of a second magnetic material over said dielectric layer, the second magnetic material being of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anisotropy field than the 

1. A READ/WRITE TRANSDUCER COMPRISING: A PAIR OF MAGNETIC POLE-PIECES AND READ-WRITE CONDUCTOR MEANS INDUCTIVELY COUPLED TO SAID POLE-PIECES AND PASSING THEREBETWEEN, EACH POLEPIECE INCLUDING THE SUPERPOSITION OF: A FIRST MAGNETIC LAYER MEMBER CLOSE TO SAID CONDUCTOR MEANS, A MAGNETOSTATICALLLY DECOUPLING LAYER MEANS, OVER SAID FIRST MAGNETIC LAYER MEMBER, AND A SECOND MAGNETIC LAYER MEMBER OF SUBSTANTIALLY LOWER PERMEABILITY, SUBSTANTIALLY HIGHER SATURATION FLUX DENSITY AND APPRECIABLY HIGHER VALUE OF ANISTROPY FIELD THAN SAID FIRST MAGNETIC LAYER MEMBER.
 2. A read/write transducer according to claim 1, wherein said magnetostatically decoupling layer means is comprised of a non-magnetic dielectric material.
 3. A read/write transducer according to claim 1, wherein said magnetostatically decoupling layer means comprises a stack of thin films of alternating magnetic materials one of which is of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anisotropy than the other one.
 4. A read/write transducer according to claim 1, wherein said first magnetic layer member consists of a stack of relatively insulated thin magnetic films.
 5. A read/write transducer according to claim 4, wherein said second magnetic layer member also consists of a stack of relatively insulated thin magnetic films.
 6. A read/write transducer according to claim 1, wherein the material of said first magnetic layer member is an iron-nickel alloy and the material of said second magnetic layer member is selected from the group consisting of iron, cobalt and the iron-cobalt alloys.
 7. In a read/write magnetic transducer comprising a pair of magnetic pole-pieces and read-write conductor means inductively coupled to the pole-pieces and passing therebetween, a composite pole-piece structure comprising a stack of magnetic maTerial thin films wherein a first plurality of said films close to the said conductor means are relatively insulated and of a first magnetic material, a second plurality of said films over the first plurality are directly contacting each other and alternate first and second magnetic materials, and a third plurality of said films are relatively insulated and of the said second magnetic material over the second plurality of thin films, and wherein the said second magnetic material is of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anisotropy field than the first magnetic material.
 8. In a read/write magnetic transducer comprising a pair of magnetic pole-pieces and read-write conductor means inductively coupled to the pole-pieces and passing therebetween, a composite pole-piece structure comprising a first plurality of relatively insulated thin film of a first magnetic material close to said read-write conductor means, a nonmagnetic dielectric material layer over first plurality of thin films, and a second plurality of relatively insulated thin films of a second magnetic material over said dielectric layer, the second magnetic material being of substantially lower permeability, substantially higher saturation flux density and appreciably higher value of anisotropy field than the first magnetic material. 